For aluminium production, technology based on a carbothermic process is promising and offers the prospect of an alternative to the Hall-Héroult electrolytic technology. A successful carbothermic process would have the potential to reduce capital investment requirements by 50 to 70% and operating costs by 25 to 35% compared to the current electrolytic route. Also, the problem of fluoride emission would be obviated, while the quantity of generated carbon containing gases would be substantially lower than for electrolytic production of aluminium.
Attempts to produce aluminium by a carbothermic process have been made for in excess of 100 years. However, optimisation of a carbothermic process to enable successful commercial production of aluminium is yet to be achieved. Processes investigated to this stage (other than the applicant's) require temperatures in excess of 2,000° C. and accurate control of reactants and products at different complex stages. The stages include:                (a) reaction of alumina and carbon to produce aluminium carbide at above 2,000° C.;        (b) reaction of the aluminium carbide with alumina to produce aluminium metal at above 2,150° C.; and        (c) separation of the aluminium from remaining materials.        
Challenges to be met in such carbothermic process include successfully recovering the high level of volatilized aluminium, reducing the level of refractory loss, the difficulties of transferring materials between stages and the problem of generation of a high volume of carbon monoxide. Such issues are inevitable at operating temperatures as high as 2,000 to 2,250° C.
Reactions central to the carbothermic processes are:2Al2O3+9C→Al4C3+6CO,  (1) andAl2O3+Al4C3→6Al+3CO  (2)
These reactions give the overall reaction of:Al2O3+3C→2Al+3CO  (3)
Earlier work on the production of aluminium by these reactions is illustrated by U.S. Pat. Nos. 1,219,797 and 1,222,593 both to Barnet et al; U.S. Pat. Nos. 2,090,451 and 2,255,549 both to Kruh; U.S. Pat. No. 2,755,178 to Rasmussen; U.S. Pat. No. 2,776,884 to Grunert alone; and U.S. Pat. No. 2,829,961 to Miller at al; and U.S. Pat. No. 2,974,032 to Grunert.
More recent work has been directed to reacting alumina and carbon in a molten bath having a molten slag of aluminium carbide and alumina. The molten bath usually operates with two zones, in a first of which aluminium carbide is generated, and a second to which the carbide passes to be reacted with alumina to produce metallic aluminium. This work is illustrated by U.S. Pat. No. 4,385,930 to Persson; U.S. Pat. No. 6,440,193 to Johansen et al; U.S. Pat. No. 6,475,260 to LaCarmera; U.S. Pat. No. 6,530,970 to Lindstad; U.S. Pat. No. 6,849,101 to Fruehan et al; and U.S. patent application publication 2006/0042413.
Also of interest are the publications: “Carbothermal Production of Aluminium” by Motzfeldt et al, published in 1989 by Aluminium-Verlag GmbH of Dusseldorf, Germany; and “Aluminium Carbothermic Technology” submitted to U.S. Department of Energy under Cooperative Agreement Number DE-FC36-00ID13900 by MJ Bruno and Alcoa Inc, and dated 31 Dec. 2004.